1. Field of the Invention
The present invention relates to developing apparatuses for electrophotographic image forming apparatuses used in copiers, printers, facsimiles, and machines that combine these functions. In particular, the present invention relates to one-component type developing apparatuses in which a spherical toner is formed into a layer on a toner holding member and, in a development region, the spherical toner is developed onto an electrostatic latent image holding member.
2. Description of Related Art
In recent years, electrophotographic developing apparatuses can be classified roughly into one-component development systems in which only a toner is used and two-component development systems in which a toner and a carrier are used. In the one-component development systems, a toner is supplied from a toner supply member to a toner transporting (holding) member, and after the toner is formed into a thin layer by a layer regulating member, the toner is developed onto an electrostatic latent image on a photoreceptor, which is an electrostatic latent image holding member.
On the other hand, in the two-component development systems, a magnetic brush is formed, using a developer constituted by a toner and a carrier that is constituted by magnetic particles, on a magnet roller containing a magnet, and then the toner is developed while the magnetic brush is rubbed against the electrostatic latent image on the photoreceptor, which is the electrostatic latent image holding member.
In the one-component development systems, a toner replenishment mechanism is simpler than that in the two-component development systems, and a mechanism for mixing and stirring the toner and the carrier and a toner concentration sensor for detecting the mixing ratio of the toner are not necessary. Thus, the one-component development system is suitable for miniaturization of the apparatuses. On the other hand, since the one-component development systems do not use a carrier as a charging member, the toner cannot be charged uniformly, and thus there are problems such as fogging due to adhesion of the toner to non-image portions, a reduction in the image density in solid image portions, and the occurrence of uneven image density.
Furthermore, there is a problem of the occurrence of white stripes and black stripes in that the toner transporting member formed from an elastic member and the toner supply member formed from a foam member partially wear away, and thus portions of lighter and darker density appear on an image corresponding to the wear portions.
As the toner transporting member, a development roller made by forming silicone rubber or urethane rubber, which is an elastic member, on a metal shaft often is used. From the viewpoint of imparting chargeability to the toner, silicone rubber material is often used.
Moreover, as the toner supply member, a toner supply roller made by foaming urethane resin on a metal shaft is used, and a method of forming projections and depressions on the surface of the roller, by controlling the foaming conditions, and transporting the toner to the development roller often is used. However, since urethane resin has a low ability to charge the toner, when a toner having a low amount of charge is developed, there is the problem of fogging in which the toner adheres to non-image portions and the problem of an increase in the image density due to the low amount of charge.
As a measure for improving that toner-charging ability, a developing apparatus in which a foamed silicone rubber is used as the supply roller is proposed in JP H11-327282A. With this configuration, the problems of fogging and an increase in the image density can be avoided because of the chargeability of silicone rubber.
Moreover, an example of preventing clogging in a supply roller made of a foamed silicone rubber or urethane rubber is proposed in JP 2003-13944A. According to this configuration, clogging of the toner in the cells of the foam is prevented by making the shape of the toner spherical.
However, when using the supply roller formed from a silicone rubber foam as in the configuration in JP H11-327282A, although the toner-charging ability is improved, wear of the supply roller is exacerbated promoted more than when using a urethane rubber foam.
In particular, in the case of a toner having an irregular shape, polishing action by the toner is increased, and thus wear of the supply roller is increased. Consequently, the ability to transport the toner to the development roller is decreased, and thus problems of uneven image density and poor image-density reproduction due to variations in the thickness of the toner layer occur. Moreover, the contact pressure between the supply roller and the development roller is reduced, and thus problems of fogging due to a reduction in the amount of charge and an increase in the image density also occur.
Moreover, there is the problem of black stripes in that the hardness of the supply roller is increased because cell portions of the foam material are filled up with the toner, and wear of the development roller is promoted, resulting in an increase in the image density in the area of that wear.
That is to say, when using the supply roller formed from a silicone rubber foam, wear of the supply roller and the development roller is promoted, and thus there is a problem in that the life is reduced.
Moreover, when using a silicone sponge made of a foam material and a toner that is made spherical as in JP 2003-13944A, the polishing action of the toner is reduced due to the shape effect, and thus wear of the supply roller and the development roller can be prevented. Moreover, clogging of the toner in the supply roller hardly occurs, and thus hardening of the supply roller due to the toner can be prevented.
However, since the toner that is made spherical has fewer points at which it is brought into contact with the development roller and the supply roller than the toner having an irregular shape, the number of times the toner is charged is decreased. The chargeability is reduced and thus the phenomenon of fogging occurs.
Moreover, with this toner, the transporting ability deteriorates because the image force to the development roller is decreased due to a reduction in the chargeability and the intermolecular force with respect to the development roller is decreased due to the spherical shape, so that a problem of solid-image reproducibility occurs.